The post about the botched Costello assassination raised another question.
Why didn't Costello simply quietly remove Genovese from the ranks of the living as soon as Genovese had returned to NY?

Like Vito Corleone, was Frank slipping?

Geneovese was nothing but trouble for Costello almost as soon as he got back. Now the rules say you aren't supposed to get rid of a made man without cause but the rules are made to be broken, especially by the bosses. And anyway Genovese had provided plenty of cause right up until the time he made his move.

Costello was never on "the muscle end of the business" but all the same shouldn't a savvy boss like Costello have recognized the threat Vito posed?

I think that by that time, Costello just lost his stomach for the business. He was probably quite content having framed Vito and having him die in prison. He might have thought that to be a fate worse than death.

When Genovese came back to America in 1945, I think Costello was at the height of his power, and could not fathom that anyone, even Genovese would make a move on him. Costello wasn't by nature a violent man so even though he probably suspected Genovese aspired to be Boss once again, there was no reason to ignite a potentially volatile situation by whacking Genovese, who was popular among certain elements of the family in his own right.

Although he was a white collar guy, Costello still at that point had the support of Willie Moretti, his cousin, who had an army of goons at his disposal. Combined with Costello's white collar criminal base plus his political and law enforcement contacts, Genovese knew that any move on Costello would have been suicidal. Genovese wisely bided his time and struck when Costello was at his weakest.

I suppose you could view it as a mistake on Costello's part to not kill Genovese when he still had the muscle behind him.

Exactly. That's my question. Maybe not get rid of Vito in 1946 but certainly by 1950 or so Costello should have picked up on rumblings of discontent and the fact that Vito thought he should be boss.